Technical Field
Embodiments of the present invention relate to an organic light emitting device that is able to secure a flatness of an organic layer and prevent a smear of an outer portion of an active area.
Discussion of the Related Art
Recently, liquid crystal display (LCD) devices have been widely used as flat panel display devices. The LCD devices use a backlight as a separate light source, and have technical limitations in brightness and contrast. To solve these problems, organic light emitting devices that self-emit light without a separate light source and have relatively better brightness and contrast than those of the LCD devices have been developed, and are being commercialized.
The organic light emitting devices are divided into a passive matrix type and an active matrix type depending on a driving mode. Hereinafter, an active matrix type organic light emitting device according to the related art will be described with reference to drawings. For reference, in the present specification, the active matrix type organic light emitting device is briefly referred to as an organic light emitting device.
FIG. 1 is a view schematically illustrating a structure of a related art organic light emitting device. As shown in FIG. 1, one pixel formed in an active area (an emission area) of an entire area of the organic light emitting device is illustrated, and a plurality of lines and a thin film transistor (TFT) for driving an organic light emitting diode (OLED) 40 are not illustrated.
Referring to FIG. 1, the related art organic light emitting device includes a base substrate 10, a TFT array layer 20, a planarization layer 30, an OLED 40, a first protective layer 50, a second protective layer 60, a third protective layer 70, and a plastic film 80.
Since the OLED 40 has characteristics that are vulnerable to moisture, the OLED 40 should be sealed from the outside. Therefore, a plurality of the protective layers 50, 60 and 70 are formed to cover the OLED 40, thereby protecting the OLED 40 from external moisture.
The first and third protective layers 50 and 70 are each formed as an inorganic protective layer, and the second protective layer 60 is formed as an organic protective layer. The plastic film 80 is formed on the first to third protective layers 50, 60 and 70, and an OLED panel is encapsulated by the plastic film 80.
FIG. 2 is a view schematically illustrating a structure of an organic emission layer.
Referring to FIG. 2, the OLED 40 is formed in a structure where a hole injecting layer (HIL) 42, a hole transport layer (HTL) 43, an organic emission layer (EML) 47, an electron transport layer (ETL) 46, and an electron injecting layer (EIL) 45 are sequentially stacked. An anode electrode 41 is formed under the HIL 42 to inject a positive hole, whereas a cathode electrode 44 is formed on the EIL 45 to inject an electron.
When an electron generated by the cathode electrode 44 and a positive hole generated by the anode electrode 41 are injected into the organic emission layer 47, the injected electron and positive hole are combined to generate an exciton. The generated exciton is dropped from an excited state to a ground state to emit light, and an image is displayed by using the emitted light.
FIG. 3 is a view for illustrating a problem in that an organic layer is thickly formed at an outer portion of an active area, in a structure of multi protective layers.
Referring to FIG. 3, among the multi protective layers 50, 60 and 70 for protecting the OLED 40 from an external factor, the first and third protective layers 50 and 70 that are inorganic protective layers are formed by using a first mask and a first chamber.
The second protective layer 60 that is an organic protective layer is formed by using a second chamber and a screen printing process using a second mask.
To describe the screen printing process, a mask is disposed on a substrate, and an organic material such as a polymer is dropped on the mask by using a nozzle equipment. The organic material is coated on the first protective layer 50 by using a squeeze bar, and then is fired in a chamber, thereby forming the second protective layer 60.
As described above, when the second protective layer 60 that is an organic layer is formed by the screen printing process, an organic layer formed at an edge of the active area is thickened (edge top).
When the second protective layer 60 that is an organic layer is thickly formed at the edge of the active area, a smear occurs, thereby degrading a display quality of an image.